The youthful Foxes travelled to the south coast in good form, unbeaten in 11 matches and having won two of their three group stages fixtures to reach the last eight of this year’s competition.
Three changes from the side that drew away to Arsenal in U18 Premier League South saw Harvey Godsmark-Ford, Sammy Braybrooke and Will Alves come in for Joe Wormleighton, Kartell Dawkins and Jack Butterfill.
After a tight opening, the Foxes took the lead on the stroke of half-time, when the Seagulls’ Ruairi McConville turned the ball into his own net.
McConville atoned for his error in the second period, netting just after the hour-mark to set up an interesting final 30 minutes at AMEX Elite Football Performance Centre.
As extra-time drew closer, it was the visitors who struck, with substitute Zac Booth finding the all-important second goal to secure a place in the last four of the competition and stretch the unbeaten run to a 12th game.
City turn their attention back to league matters on Saturday 19 February, taking on Tottenham Hotspur at LCFC Training Ground (12pm GMT kick-off).
The details...
City: Chibueze; Grist, Wilson-Brown, Godsmark-Ford; Lindsay, Cartwright, Braybrooke, Kutshienza (Booth 61'); Alves, Cover, Popov (Butterfill 89').
Subs not used: Lewis, Cook, Weeks.
Goals: McConville o.g. 45', Booth 75'.